The field of the disclosure relates generally to satellite systems and, more particularly, to determining propellant mass and system center of gravity for satellites.
Accurate knowledge of the center of gravity of the propellant tanks in known satellite systems, such as for example navigation satellites that require highly accurate positioning, also require accurate knowledge of the center of gravity for the entire satellite. With this information the phase center displacement of the navigation system antenna with respect to the satellite's center of gravity may be determined. Reporting accurate location of the phase center of the navigation antenna to a user minimizes the User Range Error (URE), thus increasing the precision of the position reported to the user.
Moreover, accurate knowledge of propellant content in a propellant tank is critical for all satellite missions, i.e., navigation, commercial, scientific or military. With this information all mission lifetimes can be maximized to operate up to full operational and design capabilities, thus maximizing return on investment for a satellite customer. In known spin-stabilized satellites, the artificial gravitational field generated therein enables the use of conventional methods for determining propellant content in the spacecraft. For example, some known spin-stabilized satellites determine the amount of liquid propellant remaining by measuring a height of the liquid within the spacecraft propellant tank. Alternatively, a liquid pressure at the bottom of the tank may be gauged. However, conventional gravity based fuel measurement methods are inappropriate for three-axis stabilized spacecraft due to the absence of a gravitational field of sufficient magnitude.
Accordingly, alternative methods have been developed to estimate the amount of propellant remaining within the propulsion systems of three-axis stabilized spacecraft that operate in zero and/or low gravity environments. Some known satellites use a method that includes monitoring changes in the absolute pressure within the propellant tank to thermodynamically deduce the volume of propellant remaining in the tank. However, the narrow range of absolute pressures within the propellant tanks often results in an unacceptable measurement error. For example, in the case of some known geosynchronous satellites, up to eighty percent of the initial propellant loaded on the spacecraft may be expended to attain the final operational orbit. Thus, a low percentage of the initial propellant load remains at the onset of the operational life of the satellite. Since the absolute pressure variance of the remaining propellant load is relatively small during the operational life of the satellite, predictions of the propellant remaining onboard the satellite are prone to significant error.
Some other known satellite systems that operate in a zero or low gravity environment may employ the “bookkeeping” approach. Specifically, the mass of propellant initially loaded into the spacecraft is recorded. As spacecraft thrusters are fired during launch operations and/or during station keeping maneuvers, the amount of propellant burned during such maneuvers is estimated. The amount of propellant remaining is calculated as the difference between the propellant initially loaded into the spacecraft and that estimated to have been expelled. However, uncertainty with respect to temperature and pressure leads to errors in the determination of the actual quantity of propellant consumed during the aforementioned maneuvers. Such errors tend to accrue over the operational life of the spacecraft, which increases the difficulty in making predictions as to the end of the operational life of the spacecraft. Further, the bookkeeping approach is incapable of accurately accounting for propellant leakage. In practice, the bookkeeping method may yield erroneous end of life predictions in the range of one year for missions of approximately ten years or more. Hence, in such known systems, the one year inaccuracy results in the launch of a replacement spacecraft one year in advance of the nominal launch date under an accurate prediction. Maintaining a replacement satellite in orbit during this uncertainty period is inefficient and tends to increase costs.
The uncertainty in forecasting the probable time of spacecraft propellant expiration and termination of the useful life, as well as inaccurately determining a center of gravity for the spacecraft, tends to complicate mission planning and may cause critical inaccuracies for the satellites users and/or customers.